The present invention relates to echo cancellers for use in telephone installations. A major application lies in installations likely to include portable terminals or "hands-free" terminals. Such installations include, in particular, digital cellular radiotelephone systems, videophone terminals operating in narrow band (8 kHz) or in enlarged band (16 kHz), and terminals for the conventional wire network.
The principle of an echo canceller is illustrated in FIG. 1 of the present application. FIG. 1 shows a "near" terminal which may be a teleconferencing set, a "hands-free" telephone handset, etc. The signal x travelling over a receive line LR and coming from a remote terminal 8 is amplified by an amplifier 10 and then broadcast by a loudspeaker 12. The soundwaves coming from the loudspeaker 12 are transmitted by acoustical coupling (represented by path 13) to one or more microphones 14 that are intended for picking up speech of a near speaker. The microphone 14 is connected to an amplifier 16 and the output signal z is transmitted over a send line LE to the remote terminal. The "remote" speaker placed at the remote terminal 8 will consequently hear not only any intended speech, but also an echo of his or her own speech after a delay due to the transmission time via the channels LE and LR, and disturbed by the transfer function of the acoustical path 13.
The echo canceller includes an adaptive filter FA which receives the input signal x and whose output is applied to the subtractive input of a subtracter 18 whose additive input receives the output from the amplifier 16. The coefficients of the filter FA are automatically adapted responsive to the error signal e sent over the send line LE and equal to the difference between the signal z (constituted by the echo when there is no useful signal) and the output from the filter. When the speaker at the near terminal is not speaking, the error signal is constituted merely by residual echo. Often the echo canceller is provided with a detector for detecting the presence of near speaker speech, referred to as a double speech detector (DIP), which stops or slows down adaptation of the filter FA while the local speaker is speaking, so as to prevent the filter from being disturbed by local speech.
In general, the filters of echo cancellers in telephone installations are digital adaptive transversal filters using a simple algorithm such as the gradient algorithm or more commonly the normalized stochastic gradient algorithm, referred to as NLMS.
A major problem in implementing an echo canceller is due to the large number of coefficients required for taking account of the length of the impulse response of the path 13 throughout the telephone band. Several thousands of coefficients are required at the usual sampling frequency of 8 kHz (narrow band) or of 16 kHz (enlarged band). To keep the volume of computation compatible with the computation power of available digital signal processors, proposals have been made to subdivide the passband in which echo cancelling is to be performed into a plurality of sub-bands, each processed by a path having a sub-band analysis filter, a canceller allocated to the sub-band, and a sub-band synthesis filter.
This approach encounters difficulties. Either the filters are designed so that the sub-bands are separate, thereby avoiding spectrum aliasing effects in each sub-band, but causing gaps in the spectrum of the signal as reconstructed by combining the outputs of the synthesis filterbank. Or else, a sub-band-feeding sub-sampling frequency is adopted which is greater than the critical frequency for sub-sampling or decimation in order to form guard bands which avoid spectrum aliasing, but that considerably increases the computation speed required. Or else the sub-bands are allowed to overlap and account is taken of the contribution of adjacent sub-bands in the path allocated to any one sub-band. This solution (U.S. Pat. No. 4,956,838) gives results that are satisfactory, but it suffers from the drawback of considerably complicating the structure of an echo canceller.
It is also known that the algorithms conventionally used in echo cancellers, and in particular the stochastic gradient algorithm, allows significant residual echo to remain in a noisy environment.
A certain number of filtering algorithms are known that make it possible, in theory, to reduce the residual error. In particular, the recursive least squares algorithm or RLS algorithm is known which gives better performance than the NLMS algorithm commonly used in echo cancelling. However it suffers from the drawback of being complex and of having zones of instability. That is why it has not been used in acoustical echo cancellers, in particular because the complexity of implementing it would require a large amount of hardware and computation times that are incompatible with the delays that are acceptable in telephone communications. For example, in the European GSM standard for digital cellular telephony, the maximum processing time allowed to a station is 0.1 s.